In a gas turbine, the combustion heat of a fuel is converted into mechanical work. The thermodynamic cycle that describes this conversion corresponds approximately to the Joule cycle.
In that context, first an oxygen-containing working gas, in practice generally air, is compressed by a compressor chamber, in the course of which it heats up from a starting temperature T1 to T2, and the pressure increases. In the second step, heat is supplied to the working gas in a combustion chamber at constant pressure by burning the admixed fuel, as a result of which the temperature rises further to T3. The compressed, heated working gas then performs mechanical work by expanding and in the process driving the turbine via blades. In so doing, the temperature drops to T4. The pressure also drops. Some of the mechanical work obtained at the turbine can be used for compression in the first stage. In a last stage, waste heat is extracted from the working gas at constant pressure by cooling, whereby the temperature drops back to T1. For the approximation of an ideal gas, the efficiency of the process is given byη=1−(T4−T1)/(T3−T2).
As a consequence of the widespread adoption of renewable energy generation in more and more industrialized countries, increased importance is placed on thermal power plants that use gas turbines. The lack of planning reliability that is naturally associated with energy generation using solar power or wind power must be equalized by sufficient reserve capacities in generation, which are also able to provide the required power as quickly as possible. In this case, power plants that are operated using gas turbines have a marked advantage, due to the volatility of the fuel used, over the thermodynamically more sluggish coal-fired power plants or even nuclear reactors.
The efficiency of a gas turbine as a quotient of the energy generated over the total energy content of the fuel used is well below 50%, even in a modern installation, since the heat supplied by combustion is discarded as waste heat, and thus the energy content of the waste heat after expansion of the working gas is no longer used.
However, the efficiency can be increased by using this waste heat, for example by using the waste heat in a second circuit to operate a steam turbine of a second thermal power plant (what is referred to as “combined cycle” technology). This allows the efficiency to be improved by the degree of waste heat that can be supplied to the steam turbine. However, this improvement in efficiency implies an increase in the system complexity, since it is now necessary to connect the entire steam circuit to the gas turbine and to coordinate it with the latter in terms of control, all of which increases the investment costs for a plant. In addition, many existing thermal power plants with gas turbines cannot readily be retrofitted with a steam circuit due to the dimensions of the components of the latter.
In this context, one possible further solution can be to use the waste heat of the exhaust gas of the gas turbine, by integration into a recuperator process, to further heat the working gas pre-compressed in the first step before the combustion heat of the fuel is supplied. Since in many gas turbines the temperature T2 of the pre-compressed working gas is below the waste heat temperature T4, in the ideal model the quantity of heat corresponding to this difference T4-T2 need not be supplied to the working gas by the energy content of the fuel but can be saved, which leads to a corresponding increase in efficiency.
In this context, one unsolved problem arises from the technical implementation of the integration in particular of an existing gas turbine system into a recuperator process, wherein the recirculation of the working gas from the heat exchanger is of particular significance here.